EP0307721B1 - Optical heterodyne receiver with a six-port fibre coupler - Google Patents

Abstract

In a conventional optical heterodyne receiver with a standard six- port fibre coupler, a loss of sensitivity occurs compared with an optical heterodyne receiver with a 90 DEG hybrid and, furthermore, laser amplitude noise is inadequately suppressed. Therefore, according to the invention, the receiver with a six-port fibre coupler (K) is designed as a pair of balanced receivers, so that the photocurrent amplifiers connected behind the photodiodes (PD1, PD2, PD3), through suppression of the DC components in the photocurrent, receive only the differences in current between the photocurrents of in each case two photodiodes. <IMAGE>

Description

The invention relates to an optical superimposed receiver with inputs for a received signal and the signal of a local laser, with a triple-fiber coupler connected to the inputs as an optical 120 ° hybrid and with optically coupled to the outputs of the triple-fiber coupler, the output signals of which via a first to third photo current amplifier and a first to third signal demodulator are applied to three inputs of a summer and that the data signal can be taken from the signal output thereof.

Analogous to an electrical superimposed receiver, an optical superimposed receiver can be constructed, in which an optical received signal is mixed with the signal from a local laser. The mixing takes place in a photodiode as an optoelectric converter, the photodiodes thus emit an electrical signal directly at an intermediate frequency level. The intermediate frequency is the difference frequency between the frequency of the received signal and the oscillation frequency of the local laser. In order to be able to transmit the current data rates of 565 Mbit / s or even the future data rate of 2.5 Gbit / s, you have to choose a very high intermediate frequency, which is usually in the gigahertz range. However, this results in the need to use highly sensitive amplifiers with a very high bandwidth and cutoff frequency in the intermediate frequency level. According to Electronics Letters from 09/12/85, VOL 21, No. 19, pages 867 and 868, with multi-port reception, there is the possibility of using optical superimposed receivers with very low intermediate frequencies, so that intermediate frequency amplifiers can be used with a bandwidth which corresponds approximately to the base bandwidth for homodyne or direct reception.

Digest No 1985/30 of the IEE Colloquium on Advances in Coherent optical Devices and Techniques, London 1985, pages 11/1 to 11/5 describes a phase-insensitive optical homodyne receiver with a three-port receiver using a 3x3 fiber coupler as a 120 ° hybrid because of the In non-ideal phase characteristics of this coupler, there is a loss of sensitivity of 1.8 dB compared to the optical heterodyne receiver with a two-port receiver according to Electronics Letters, which requires a comparatively very precise 90 ° hybrid. A broadband optical phase diversity homodyne receiver using a triple fiber coupler is also described in SPIE VOL 630 Fiber Optics (Sira) 1986, pp. 28-32. Three photodiodes are optically connected to the outputs of the triple fiber coupler, in each of which the mixture between components of the same polarization of the received light and the light of the local laser takes place. In addition, an electrical component corresponding to the mixture product is generated by the photodiodes and output to downstream photocurrent amplifiers. The photo currents generated are phase-shifted from one another by 120 ° and are converted into baseband signals in downstream demodulators for the modulation method selected in each case. These baseband signals are combined in a three-input summer, and the demodulated data signal is thus generated.

In addition to a loss of sensitivity when known triple couplers are used, the occurrence of laser amplitude noise in the baseband signal and the direct current component in the photocurrent are disadvantageous in the known optical superposition receivers described. it prevents or significantly complicates the use z. B. a high impedance photocurrent amplifier.

The object of the present invention is therefore to improve the signal-to-noise ratio in optical heterodyne receivers of the type mentioned in the introduction and to To generate amplifying photo current as DC-free as possible.

According to the invention, the object is achieved by an optical overlay receiver of the type mentioned at the outset, which is developed in accordance with the features contained in the characterizing part of patent claim 1. There is no loss of sensitivity of 1.8 dB, as occurs in the conventional phase diversity receiver with an optical 3 x 3 coupler compared to the phase diversity receiver with two ports. Of particular advantage in the optical superposition receiver according to the invention is the suppression of the direct components in the photocurrent of the photodiodes together with the suppression of the laser amplitude noise, so that a significant improvement in sensitivity can be achieved without a significant increase in expenditure.

An embodiment with particularly little effort is described in claim 2.

A preferred embodiment of the optical superimposition receiver according to the invention is explained in more detail in patent claim 3, expedient designs of the current mirrors used in this optical superimposition receiver are specified for claims of discrete construction in claims 4 and 5 and for integrated construction in claims 6 and 7.

The invention will be explained in more detail in the present case with reference to exemplary embodiments shown in the drawing.

Fig. 1

den prinzipiellen Aufbau eines erfindungsgemäßen optischen Überlagerungsempfängers,

Fig. 2

einen erfindungsgemäßen optischen Überlagerungsempfänger mit einem, vorgespannte Fotodioden enthaltenden Eingangsteil,

Fig. 3

im Eingangsteil der Fig. 2 verwendbare Stromspiegel mit diskreten Elementen und

Fig. 4

im Eingangsteil der Fig. 2 verwendbare Stromspiegel in integrierter Technik.

It shows

Fig. 1

the basic structure of an optical superimposition receiver according to the invention,

Fig. 2

an optical superimposition receiver according to the invention with an input part containing biased photodiodes,

Fig. 3

Current mirrors with discrete elements and which can be used in the input part of FIG

Fig. 4

Current mirrors that can be used in the input part of FIG. 2 in integrated technology.

1 is constructed as a push-pull phase diversity receiver and contains a triple fiber coupler K in the optical input part. The first optical input connection of the triple fiber coupler K is connected to a source for a reception signal ES, to which the second input connection is the local laser oscillator LO is connected, the third input connection remains unconnected. The three output connections of the triple fiber coupler are optically connected to three photodiodes PD1, PD2, PD3. These photodiodes are connected in push-pull with respect to the downstream photocurrent amplifiers V1, V2, V3, so that in each case a differential photocurrent flows into the input connections of the photocurrent amplifiers. For this purpose, the anode connection of the first photodiode is connected to the cathode connection of the second photodiode and at the same time to the input connection of the first photocurrent amplifier V1. Accordingly, the anode connection of the second photodiode PD2 is connected to the cathode connection of the third photodiode PD3 and to the input connection of the second photocurrent amplifier V2. The anode connection of the third photodiode PD3 is connected in an analogous manner to the cathode connection of the first photodiode and to the input connection of the third photocurrent amplifier V3. If one designates the currents flowing through the first, second and third photodiodes PD1, PD2, PD3 with I₁, I₂, I₃ then the difference I₁-I₂ flows into the input connection of the first photocurrent amplifier, the difference I₂-I₃ into the input connection of the second photocurrent amplifier V2 and in the input terminal of the third photocurrent amplifier V3 the difference I₃-I₁. The photocurrent amplifiers in the form of transimpedance or high-impedance amplifiers are designed as current-voltage converters, so that voltages corresponding to the photocurrent differences arise at their outputs. The photocurrent amplifiers V1, V2, V3 each have a first, second, third demodulator DM1, DM2, DM3 for the selected one Modulation process downstream. In the exemplary embodiment, differential pulse phase jump modulation (DPSK) is used; it is also possible with appropriately modified demodulators - the reception of PSK or ASK - modulated signals. The photocurrent amplifier and demodulator each form a signal channel, with the phase of the currents flowing in the channels being shifted from one another by 120 °.

The following applies approximately for the current I _n in channel n:

wobei B die Rauschbandbreite ist.The shot noise power in channel n is:

$${\text{N}}_{\text{n}}\text{≃}\frac{\text{2nd}}{\text{3rd}}{\text{e K}}_{\text{n}}{\text{P}}_{\text{O}}\text{B,}$$

where B is the noise bandwidth.

With DPSK, the modulation takes place by keying the phase Φo. As before, demodulation in the form of an envelope demodulation takes place separately in each channel before the summation of the three channels.

With the previous method, the maximum signal-to-noise ratio in a channel is

This is a factor of 3 or 4.8 dB worse than ideal homodyne reception. The triple coupler is symmetrical, ie φ n = n · 120 ° and K _n = K. The maximum differential current is therefore of size

$${\text{I.}}_{\text{n + 1}}{\text{- I}}_{\text{n}}{\text{|}}_{\text{Max}}\text{= √}\overline{\text{3rd}}\frac{\text{2nd}}{\text{3rd}}\text{K √}\overline{{\text{P}}_{\text{s}}{\text{P}}_{\text{O}}}\text{.}$$

d. h. es ist nur 3 dB schlechter als der ideale Homodynempfang. Zudem ist das Differenzsignal gleichstromfrei und frei von eventuellem Laseramplitudenrauschen.The signal-to-noise ratio now reaches (N _new = 2N _n )

ie it is only 3 dB worse than the ideal homodyne reception. In addition, the differential signal is free of direct current and free of any laser amplitude noise.

Pin diodes have been used as photodiodes in the exemplary embodiment. The demodulators DM1 - DM3 are DPSK demodulators. The totalizer is designed as an emitter follower with three inputs. Slight asymmetries of the fiber coupler can be corrected by different coupling to the photodiodes and / or by different adjustment of the internal amplification when using avalanche multiplication diodes and / or by correction elements when forming the differential photo currents.

FIG. 2 shows an input part of an optical heterodyne receiver based on the phase diversity principle with biased photodiodes. The photodiodes PD1, PD2, PD3 each receive a bias voltage from a first or second current mirror SS1, SS2, which in turn are connected to a first or second voltage source U1, U2. The distribution of the bias voltages among the diodes is effected by the definition of the direct voltage operating points or photocurrent amplifiers V1 and V2. As with the overlay receiver according to FIG. 1, there are also the overlay receiver according to FIG. 2 the anode of the first photodiode PD1 and the cathode of the second photodiode PD2 and the input connection of the first photocurrent amplifier V1 are connected to one another, and the connection from the anode of the second photodiode PD2 to the cathode of the third photodiode PD3 and to the input connection of the second photocurrent amplifier V2 corresponds to FIG. 1. In addition, two voltage sources U1, U2 are provided, which supply the bias voltage for the photodiodes. The bias voltage is supplied via a first current mirror SS1, the first terminal 1, 1 of which is connected to the first operating voltage source U1 and the second terminal 1, 2 of which is connected to the cathode of the first photodiode PD1. This flows through the second terminal 1.2 of the current I₁, which also flows from the third terminal 1.3 of the first current mirror SS1 in the form of the current I₁ 'because of the current mirroring. The second current mirror SS2 is connected to the second operating voltage source U2 via its first connection 2, 1. The second terminal of this current mirror is connected to the anode of the third photodiode PD3, so that the current I₃ flows through this second terminal 2.2. According to the current mirror effect flows through the third terminal 2.3 of the second current mirror SS2, an equal current I₃ ', so that at the junction of the third terminals of the first and second current mirror SS1, SS2, the differential current I₁' -I₃ 'decreased and the input terminal of third photocurrent amplifier V3 can be supplied.

The construction of a current mirror from a transistor and a diode connected in parallel with its base-emitter path is known. This principle is used in FIG. 3a to implement the first current mirror SS1 by means of discrete components. The first terminals of a first and a second resistor R1, R2 are connected to the terminal 1, 1. The second connection of the first resistor is connected via a first diode D1 to the base connection of a first transistor T1 with pnp layer sequence, the second connection of the second resistor R2 is connected to the emitter connection of the first transistor T1 connected, the base connection of this transistor represents connection 1.2 and the collector connection represents connection 1.3 of the first current mirror SS1.

A corresponding implementation of the second current mirror SS2 by means of discrete components is shown in FIG. 3b. The first terminals of a third and a fourth resistor R3, R4 are connected to the terminal 2.1. The second connection of the third resistor R3 is connected via a second diode D2 to the base connection of a second transistor T2 with npn layer sequence and also to the connection 2.2 of the second current mirror SS2. The second connection of the fourth resistor R4 is connected to the emitter connection of the second transistor T2, the collector connection of this transistor represents the connection 2.3 of the second current mirror SS2.

4 shows the implementation of the current mirror SS1 and SS2 in integrated technology. Compared to the arrangement according to FIG. 3a, the diode D1 in the arrangement according to FIG. 4a is replaced by a third transistor T3. The emitter connection of this transistor represents the anode connection of the diode D1, the collector and the base connection of this transistor are connected to one another and to the base connection of a fourth transistor T1 of the pnp type and represent the connection 1.2 of the first current mirror SS1.

4b, a fifth transistor T5 of the npn type is provided instead of the second diode D2 according to FIG. 3b. The emitter connection of this transistor represents the cathode connection of the second diode D2, while the anode connection is formed by the interconnected collector and base connections of the fifth transistor T5 and thus the connection 2.2. represent the second current mirror SS2. The connection 2, 3 is formed by the collector connection of a sixth transistor of the NPN type.

When implemented in integrated technology, adjacent transistor elements have very uniform properties, so that in this case the emitter resistors R1 ... R4 can be omitted.

Claims (7)

1. Optical heterodyne receiver having inputs for a reception signal and the signal of a local laser, having an at least approximately symmetric six-port fibre coupler (K), connected to the inputs, as an optical 120° hybrid and having photodiodes (PD1, PD2, PD3) optically coupled to the outputs of the six-port fibre coupler, the output signals of which photodiodes are applied via a first to third photocurrent amplifier (V1, V2, V3) and a first to third signal demodulator (DM1, DM2, DM3) to three inputs of a summator (SUM), and in that the data signal (DA) can be derived from the signal output thereof, characterised in that the photodiodes (PD1, PD2, PD3) are connected in each instance in push-pull with respect to the downstream photocurrent amplifiers (V1, V2, V3), so that in each instance the difference of the photocurrents of two photodiodes is present at the inputs of the photocurrent amplifiers (V1, V2, V3).
2. Optical heterodyne receiver according to Claim 1, characterised in that the anode connection of the first photodiode (PD1) is connected to the cathode connection of the second photodiode (PD2) and to the input connection of the first photocurrent amplifier (V1), in that the anode connection of the second photodiode (PD2) is connected to the cathode connection of the third photodiode (PD3) and to the input connection of the second photocurrent amplifier (V2), and in that the anode connection of the third photodiode (PD3) is connected to the cathode connection of the first photodiode (PD1) and to the input connection of the third photocurrent amplifier (V3).
3. Optical heterodyne receiver according to Claims 1 or 2, characterised in that to a first operating voltage connection (U1) there is connected the first connection (1, 1) of a first current reflector (SS1), in that the second connection (1, 2) thereof is connected to the cathode connection of the first photodiode (PD1), in that the anode connection of the first photodiode (PD1) is connected to the cathode connection of the second photodiode (PD2) and to the input connection of the first photocurrent amplifier (V1), in that the anode connection of the second photodiode (PD2) is connected to the cathode connection of the third photodiode (PD3) and to the input connection of the second photocurrent amplifier (V2), in that a second current reflector circuit (SS2) is provided, the first connection (2, 1) of which is connected to a second operating voltage source (U2),
   in that the second connection (2, 2) of the second current reflector circuit (SS2) is connected to the anode of the third photodiode (PD3), and in that the third connections (1, 3; 2, 3) of the first and of the second current reflector (SS1, SS2) are connected to one another and to the input of the third photocurrent amplifier (V3).
4. Optical heterodyne receiver according to Claim 3, characterised in that the first current reflector (SS1) includes a first and a second resistor (R1, R2), the first connections of which are connected to one another and to the first connection (1, 1) of the first current reflector (SS1),
   in that the second connection of the first resistor (R1) is connected via a first diode (D1) to the base connection of a first transistor (T1) of the pnp type and to the second connection (1, 2) of the first current reflector (SS1),
   in that the second connection of the second resistor (R2) is connected to the emitter connection of the first transistor (T1), and in that the collector connection of the first transistor (T1) is connected to the third connection (1, 3) of the first current reflector circuit (SS1).
5. Optical heterodyne receiver according to Claim 3, characterised in that the second current reflector (SS2) includes a third and a fourth resistor (R3, R4), the first connections of which are connected to one another and to the first connection (2, 1) of the second current reflector (SS2),
   in that the second connection of the third resistor (R3) is connected via a second diode (D2) to the base connection of a second transistor (T2) of the npn type and to the second connection (2, 2) of the second current reflector circuit (SS2), in that the second connection of the fourth resistor (R4) is connected to the emitter connection of the second transistor (T2), and in that the collector connection of this transistor (T2) is connected to the third connection (2, 3) of the second current reflector circuit (SS2).
6. Optical heterodyne receiver according to Claim 4, characterised in that as diode there is provided a third transistor (T3), the emitter connection of which is connected to the second connection of the first resistor (R1) and the collector and base connections of which are connected to one another, to the base connection of a fourth transistor (T4) and to the second connection (1, 2) of the first current reflector (SS1), and in that the third and the fourth transistor (T3, T4) are pnp transistors.
7. Optical heterodyne receiver according to claim 5, characterised in that as diode (D2) there is provided a fifth transistor (T5), the emitter connection of which is connected to the second connection of the third resistor (R3), and the base and collector connections of which are connected to one another, to the base connection of a sixth transistor (T6) and to the second connection (2, 2) of the second current reflector (SS2), and in that the fifth and the sixth transistor (T5, T6) are npn transistors.

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